For me, 28-105 is too narrow on the short end.
Prior to the K-1, I was using a K-5 with Pentax' DA 16-85/3.5-5.6.
HD Pentax-DA 16-85mm F3.5-5.6 ED DC WR Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database
Something equivalent to that would have a focal range of about 24-128mm. Once upon a time, there was a Tamron 24-135, but that's difficult to obtain on the 2nd hand market.
Now I'm using an (old) Pentax FA 24-90/3.5-4.5 with fairly decent results.
SMC Pentax-FA 24-90mm F3.5-4.5 AL [IF] Reviews - FA Zoom Lenses - Pentax Lens Reviews & Lens Database
For me, the missing length is not as big a problem as the reduced wideness of a 28mm lens would have been.
A pity, a real pity that Pentax decided to come up with a 28-105, when both competitors have 24-120mm lenses as better kit lenses for their full-frame DSLRs.
And another pity that Sigma continues to ignore the Pentax market by not producing the new 24-105 "art" lens with K bayonet.